1. Technical Field
The subject matter described herein relates to a polymer-stabilized, low clearing point, thermotropic liquid crystal device. Implementations of such devices have application in passive or active light-regulating and temperature-regulating films, materials and devices, including construction materials.
2. Description of the Related Art
The problem of controlling the flow of radiant energy, e.g., light and heat, in particular in applications such as regulating solar heat gain in buildings and in other applications has previously been addressed using many optical and infrared methodologies. Photodarkening materials have been used for decades, for example, in sunglass lenses, to selectively attenuate incoming light when stimulated by ultraviolet (UV) radiation. When incorporated into windows, such materials can be used to regulate the internal temperature of a structure by darkening to attenuate bright sunlight, and by becoming transparent again to allow artificial light or diffuse daylight to pass through unimpeded. Such systems are passive and self-regulating, requiring no external signal other than ambient UV light in order to operate. However, because they are controlled by UV light rather than by temperature, such systems are of limited utility in temperature-regulating applications. For example, they may block wanted sunlight in cold weather as well as unwanted sunlight in hot weather. They also do not function if placed behind a UV-blocking material such as the transparent, spectrally-selective and low-emissivity coatings that are common in the window industry.
U.S. Pat. No. 7,755,829 discloses an optical filter that can be used as a window film or other light- and heat-regulating building material that is composed of a thermotropic, low clearing point, twisted nematic, liquid crystal sandwiched between two reflective polarizers that can be used as a window film or other light- and heat-regulating building material. Similarly, in U.S. patent application publication no. 2009/0167971 by Powers et al., a thermodarkening filter composed of a low clearing point liquid crystal sandwiched between two absorptive polarizers is disclosed as a component of building materials, e.g., a window film. In addition, U.S. patent application publication nos. 2010/0045924 and 2010/0259698 by Powers et al. disclose thermotropic, light-regulating liquid crystal devices that do not require polarizing substrates. Further, U.S. patent application publication no. 2010/0045924 by Powers et al. describes some methods of manufacturing this filter technology in which the thermotropic liquid crystal is polymer-stabilized.
The mixing of liquid crystal (LC) components to obtain desired properties, such as particular values of birefringence, clearing point, crystallization point and melting point, (e.g., for use in video displays), has been practiced for decades and has been well described in the prior art. The mixing and curing of stabilizing polymers or polymer networks within a liquid crystal (e.g., for use in flexible liquid crystal devices) has also been well described. However, there are no prior formulations for thermotropic, low clearing point, polymer-stabilized liquid crystals suitable for use in twisted-nematic, thermally switched, light-regulating and heat-regulating devices.
The doping or stabilization of liquid crystal mixtures with polymers—under various nomenclature including, for example, polymer-dispersed liquid crystal (PDLC), polymer-enhanced liquid crystal (PELC), polymer-stabilized twisted nematic (PSTN), polymer network twisted nematic (PNTN), pixel isolated liquid crystal (PILC), liquid crystal dispersions, and others—is also well described. For example, U.S. Pat. No. 7,355,668 to Satyendra et al. discloses polymer-enhanced liquid crystal devices built with rigid or flexible substrates that include polymer “columns” formed between substrate films through the phase separation of a prepolymer (e.g., Norland NOA77 or NOA78 optical adhesive) and a liquid crystal (e.g., Merck E7, E48, or E31), under the influence of temperature variations. The pre-polymer and liquid crystal are mixed above the clearing point temperature of the LC, and are then cooled below the clearing point such that they phase separate and polymerize.
Another example is U.S. Pat. No. 6,049,366 to Hassan-ali et al., which discloses polymer-stabilized liquid crystals and flexible devices thereof. In this design, the phase separation and curing of the prepolymer from the liquid crystal occurs under the influence of a UV curing lamp or heat source. The liquid crystal can be any of a vast assortment of materials disclosed in the application, and the prepolymer can be any of a vast assortment as well including, but not limited to, CN934, CN94760, CN970A60, CN945B85, and UVS-500 available from Croda Resins; EBECRYL resin 284 and 810 available from UCB Chemicals; and the Photometric 6000 Series (6010, 6210, 6008, etc.) available from Henkel.
Polymer networks are also formed by phase separation and/or curing as a result of (or induced by) photo-initiation, visible light, infrared light, electric fields, lasers, charged particle beams, or chemical catalysis as well as the broad classes of solvent-induced phase separation (SIPS), thermally-induced phase separation (TIPS), and polymerization-induced phase separation (PIPS), etc. Furthermore, it is possible to impose a periodic or nonperiodic pattern on the polymer as it cures by masking and diffraction, or other similar methods, such that an image, grid, grating, holographic, or photonic material is formed, as described for example in “Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications,” by Y. J. Liu and X. W. Sun (Hindawi Publishing Corporation, Advances in OptoElectronics, Volume 2008, Article ID 684349).
Because the polymer-stabilized liquid crystal devices described above are intended as electrically operated video displays or other electro-optical devices, the devices associated with them explicitly include transparent electrode layers (e.g., indium tin oxide), and further describe, either explicitly or implicitly, liquid crystals intended to be actuated (i.e., physically reoriented) by electric fields.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention as set forth in the claims below is to be bound.